Cytokines play a role in the growth and differentiation of all cells in the body but are especially important to cells of the immune system. A category of cytokines are called interleukins, of which 18 have been identified thus far. Interleukin-2 (IL-2) is an important cytokine for the regulation of T-cell function in the immune system. Because of its important involvement in both the cellular and humoral arms of the immune system, IL-2 has been investigated extensively for a potential role in the treatment of disease. Although the primary function of IL-2 is to stimulate the growth and proliferation of T lymphocytes, IL-2 is also known to have diverse stimulatory effects on a variety of immune cells, including natural killer (NK) cells, lymphokine-activated killer (LAK) cells, monocytes, and macrophages. In regulating the immune system, IL-2 also may trigger the production of secondary cytokines, such as interferons and TNF-α, to further stimulate an immune response. Interferons, interleukins and TNF-α can be made in mass quantities through recombinant techniques for therapeutic applications.
IL-2 administration is a therapeutic treatment in cancer and other diseases. For example, IL-2 is approved for the treatment of metastatic renal cell carcinoma and melanoma. In this setting, intravenous IL-2 produces a 20% rate of remission. However the efficacy of IL-2 has been restricted by the relatively severe toxicities associated with therapeutic dosages. The native form of IL-2 exhibits toxic side effects that may include myocardial infarction, renal failure requiring dialysis, fluid retention, nausea and neuropathy. In addition, IL-2 administration is associated with generalized inflammatory changes which include the development of dose limiting capillary leak syndrome. The short half-life of i.v. administered IL-2 (about 22 minutes) requires the higher dosing that leads to toxicity.
Attempts to reduce the unwanted toxicity associated with the therapeutic use of IL-2 have focused on increasing the half-life of the molecule. This has been achieved by increasing the molecular size by linking IL-2 to another molecule such as a protein or polymer, or by linking IL-2 to a targeting molecule such as an antibody. Attempts to direct IL-2 to the site of disease by a targeting molecule have been somewhat effective and have resulted in increased levels of therapeutic efficacy, including control of malignant effusions, prevention of the growth of established tumors, and even a reduction in the size of established tumors. However, such approaches cannot be used in all anatomic locations and are not applicable to disseminated disease.
IL-2 molecules that have a mutated amino acid sequence through substitution of amino acid residues present in the wildtype IL-2 molecule have been reported to have reduced toxicity. However, such mutants are associated with altered biological function such as reduced binding affinity to forms of the IL-2 cellular receptor and altered cytokine functions, including T cell stimulation, LAK or natural killer cell activation, or secondary cytokine production. Therefore, there remains a need in the art for a low toxicity variant of IL-2 to minimize toxicities associated with treatment.